H3C S5500-SI Series User manual

Operation Manual – MSTP
H3C S5500-SI Series Ethernet Switches Table of Contents
i
Table of Contents
Chapter 1 MSTP Configuration....................................................................................................1-1
1.1 MSTP Overview.................................................................................................................1-1
1.1.1 Introduction to STP .................................................................................................1-1
1.1.2 Introduction to MSTP ............................................................................................1-10
1.2 Configuration Task List.................................................................................................... 1-15
1.3 Configuring the Root Bridge ............................................................................................1-16
1.3.1 Configuring an MST Region..................................................................................1-16
1.3.2 Specifying the Root Bridge or a Secondary Root Bridge......................................1-17
1.3.3 Configuring the Work Mode of MSTP Device.......................................................1-19
1.3.4 Configuring the Priority of the Current Device ......................................................1-20
1.3.5 Configuring the Maximum Hops of an MST Region..............................................1-21
1.3.6 Configuring the Network Diameter of a Switched Network...................................1-21
1.3.7 Configuring Timers of MSTP.................................................................................1-22
1.3.8 Configuring the Timeout Factor............................................................................. 1-25
1.3.9 Configuring the Maximum Transmission Rate of Ports.........................................1-25
1.3.10 Configuring Ports as Edge Ports.........................................................................1-26
1.3.11 Configuring Whether Ports Connect to Point-to-Point Links...............................1-27
1.3.12 Configuring the MSTP Packet Format for Ports..................................................1-28
1.3.13 Enabling the MSTP Feature................................................................................1-29
1.4 Configuring Leaf Nodes................................................................................................... 1-31
1.4.1 Configuring an MST Region..................................................................................1-31
1.4.2 Configuring the Work Mode of MSTP...................................................................1-31
1.4.3 Configuring the Timeout Factor............................................................................. 1-31
1.4.4 Configuring the Maximum Transmission Rate of Ports.........................................1-31
1.4.5 Configuring Ports as Edge Ports...........................................................................1-31
1.4.6 Configuring Path Costs of Ports............................................................................1-31
1.4.7 Configuring Port Priority........................................................................................ 1-33
1.4.8 Configuring Whether Ports Connect to Point-to-Point Links.................................1-34
1.4.9 Configuring the MSTP Packet Format for Ports....................................................1-35
1.4.10 Enabling the MSTP Feature................................................................................1-35
1.5 Performing mCheck.........................................................................................................1-35
1.5.1 Configuration Prerequisites...................................................................................1-35
1.5.2 Configuration Procedure....................................................................................... 1-35
1.5.3 Configuration Example.......................................................................................... 1-36
1.6 Configuring Digest Snooping........................................................................................... 1-36
1.6.1 Configuration Prerequisites...................................................................................1-36
1.6.2 Configuration Procedure....................................................................................... 1-37
1.6.3 Configuration Example.......................................................................................... 1-38

Operation Manual – MSTP
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1.7 Configuring No Agreement Check...................................................................................1-39
1.7.1 Prerequisites .........................................................................................................1-40
1.7.2 Configuration Procedure....................................................................................... 1-41
1.7.3 Configuration Example.......................................................................................... 1-41
1.8 Configuring Protection Functions.....................................................................................1-42
1.8.1 Configuration prerequisites...................................................................................1-44
1.8.2 Enabling BPDU Guard.......................................................................................... 1-44
1.8.3 Enabling Root Guard.............................................................................................1-45
1.8.4 Enabling Loop Guard............................................................................................ 1-45
1.8.5 Enabling TC-BPDU Attack Guard.........................................................................1-46
1.9 Displaying and Maintaining MSTP................................................................................... 1-46
1.10 MSTP Configuration Example .......................................................................................1-47

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Chapter 1 MSTP Configuration
1.1 MSTP Overview
1.1.1 Introduction to STP
I. Functions of STP
The spanning tree protocol (STP) is a protocol used to eliminate loops in a local area
network (LAN). Devices running this protocol detect any loop in the network by
exchanging information with one another and eliminate the loop by properly blocking
certain ports until the loop network is pruned into a loop-free tree, thereby avoiding
proliferation and infinite recycling of packets in a loop network.
II. Protocol Packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages,
as its protocol packets.
STP identifies the network topology by transmitting BPDUs between STP compliant
network devices. BPDUs contain sufficient information for the network devices to
complete the spanning tree computing.
In STP, BPDUs come in two types:
zConfiguration BPDUs, used to maintain the spanning tree topology.
zTopology change notification (TCN) BPDUs, used to notify concerned devices of
network topology changes, if any.
III. Basic concepts in STP
1) Root bridge
A tree network must have a root; hence the concept of “root bridge” has been
introduced in STP.
There is one and only one root bridge in the entire network, and the root bridge can
change alone with changes of the network topology. Therefore, the root bridge is not
fixed.
Upon network convergence, the root bridge generates and sends out at a certain
interval a BPDU and other devices just forward this BPDU. This mechanism ensures
the topological stability.
2) Root port
On a non-root bridge device, the root port is the port with the lowest path cost to the root
bridge. The root port is responsible for forwarding data to the root bridge. A
non-root-bridge device has one and only oneroot port. The root bridge has no root port.

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3) Designated bridge and designated port
Refer to the following table for the description of designated bridge and designated
port.
Description of designated bridge and designated port
Classification Designated bridge Designated port
For a device
The device directly connected
with this device and
responsible for forwarding
BPDUs
The port through which the
designated bridge forwards
BPDUs to this device
For a LAN The device responsible for
forwarding BPDUs to this LAN
segment
The port through which the
designated forwards BPDUs
to this LAN segment
Figure 1-1 shows designated bridges and designated ports. In the figure, AP1 and AP2,
BP1 and BP2, and CP1 and CP2 are ports on Switch A, Switch B, and Switch C
respectively.
zIf Switch A forwards BPDUs to Switch B through AP1, the designated bridge for
Switch B is Switch A, and the designated port is the port AP1 on Switch A.
zTwo devices are connected to the LAN: Switch B and Switch C. If Switch B
forwards BPDUs to the LAN, the designated bridge for the LAN is Switch B, and
the designated port is the port BP2 on Switch B.
Switch A
Switch C
Switch B CP2
BP2
CP1
BP1
AP2
AP1
LAN
Switch A
Switch C
Switch B CP2
BP2
CP1
BP1
AP2
AP1
LAN
Figure 1-1 A schematic diagram of designated bridges and designated ports
Note:
All the ports on the root bridge are designated ports.

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IV. How STP works
STP identifies the network topology by transmitting configuration BPDUs between
network devices. Configuration BPDUs contain sufficient information for network
devices to complete the spanning tree computing. Important fields in a configuration
BPDU include:
zRoot bridge ID: consisting of root bridge priority and MAC address.
zRoot path cost: the cost of the shortest path to the root bridge.
zDesignated bridge ID: designated bridge priority plus MAC address.
zDesignated port ID, designated port priority plus port name.
zMessage age: age of the configuration BPDU
zMax age: maximum age of the configuration BPDU.
zHello time: configuration BPDU interval.
zForward delay: forward delay of the port.
Note:
For the convenience of description, the description and examples below involve only
four parts of a configuration BPDU:
zRoot bridge ID (in the form of device priority)
zRoot path cost
zDesignated bridge ID (in the form of device priority)
zDesignated port ID (in the form of port name)
1) Specific computing process of the STP algorithm
zInitial state
Upon initialization of a device, each port generates a BPDU with itself as the root, in
which the root path cost is 0, designated bridge ID is the device ID, and the designated
port is the local port.
zSelection of the optimum configuration BPDU
Each device sends out its configuration BPDU and receives configuration BPDUs from
other devices.
The process of selecting the optimum configuration BPDU is as follows:
Selection of the optimum configuration BPDU

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Step Description
1
Upon receiving a configuration BPDU on a port, the device performs the
following processing:
zIf the received configuration BPDU has a lower priority than that of the
configuration BPDU generated by the port, the device will discard the
received configuration BPDU without doing any processing on the
configuration BPDU of this port.
zIf the received configuration BPDU has a higher priority than that of the
configuration BPDU generated by the port, the device will replace the
content of the configuration BPDU generated by the port with the
content of the received configuration BPDU.
2 The device compares the configuration BPDUs of all the ports and chooses
the optimum configuration BPDU.
Note:
Principle for configuration BPDU comparison:
zThe configuration BPDU that has the lowest root bridge ID has the highest priority.
zIf all the configuration BPDUs have the same root bridge ID, they will be compared
for their root path costs. If the root path cost in a configuration BPDU plus the path
cost corresponding to this port is S, the configuration BPDU with the smallest S
value has the highest priority.
zIf all configuration BPDU have the same root path cost, they will be compared for
their designated bridge IDs, then their designated port IDs, and then the IDs of the
ports on which they are received. The smaller the ID, the higher message priority.
zSelection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to
be the root bridge, with the root bridge ID being their own device ID. By exchanging
configuration BPDUs, the devices compare one another’s root bridge ID. The device
with the smallest root bridge ID is elected as the root bridge.
zSelection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Selection of the root port and designated ports

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Step Description
1 The root port is the port on which the optimum configuration BPDU was
received.
2
Based on the configuration BPDU and the path cost of the root port, the
device calculates a designated port configuration BPDU for each of the rest
ports.
zThe root bridge ID is replaced with that of the configuration BPDU of the
root port.
zThe root path cost is replaced with that of the configuration BPDU of the
root port plus the path cost corresponding to the root port.
zThe designated bridge ID is replaced with the ID of this device.
zThe designated port ID is replaced with the ID of this port.
3
The device compares the computed configuration BPDU with the
configuration BPDU on the corresponding port, and performs processing
accordingly based on the comparison result:
zIf the configuration BPDU is superior, the device will block this port
without changing its configuration BPDU, so that theport will only receive
BPDUs, but not send any, and will not forward data.
zIf the computed configuration BPDU is superior, this port will serve as the
designated port, and the configuration BPDU on the port will be replaced
with the computed configuration BPDU, which will be sent out
periodically.
Note:
When the network topology is stable, only the root port and designated ports forward
traffic, while other ports are all in the blocked state – they only receive STP packets but
do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated ports have
been successfully elected, the entire tree-shaped topology has been constructed.
The following is an example of how the STP algorithm works. The specific network
diagram is shown in Figure1-2. In the feature, the priority of Switch Ais 0, the priority of
Switch B is 1, the priority of Switch C is 2, and the path costs of these links are 5, 10 and
4 respectively.

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Switch C
Switch B
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
Switch C
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
Switch A
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
with priority 1
with priority 2
with priority 0
Switch C
Switch B
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
Switch C
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
Switch A
CP2
BP2 CP1
BP1
AP2AP1
4
10
5
with priority 1
with priority 2
with priority 0
Figure 1-2 Network diagram for STP algorithm
zInitial state of each device
The following table shows the initial state of each device.
Initial state of each device
Device Port name BPDU of port
AP1 {0, 0, 0, AP1}
Switch A AP2 {0, 0, 0, AP2}
BP1 {1, 0, 1, BP1}
Switch B BP2 {1, 0, 1, BP2}
CP1 {2, 0, 2, CP1}
Switch C CP2 {2, 0, 2, CP2}
zComparison process and result on each device
The following table shows the comparison process and result on each device.
Comparison process and result on each device

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Device Comparison process BPDU of port
after
comparison
Switch
A
zPort AP1 receives the configuration BPDU of
Switch B {1, 0, 1, BP1}. Switch A finds that the
configuration BPDU of the local port {0, 0, 0, AP1} is
superior to the configuration received message,
and discards the received configuration BPDU.
zPort AP2 receives the configuration BPDU of
Switch C {2, 0, 2, CP1}. Switch A finds that the
BPDU of the local port {0, 0, 0, AP2} is superior to
the received configuration BPDU, and discards the
received configuration BPDU.
zSwitch A finds that both the root bridge and
designated bridge in the configuration BPDUs of all
its ports are Switch A itself, so it assumes itself to
be the root bridge. In this case, it does not make
any change to the configuration BPDU of each port,
and starts sending out configuration BPDUs
periodically.
AP1: {0, 0, 0,
AP1}
AP2: {0, 0, 0,
AP2}
zPort BP1 receives the configuration BPDU of
Switch A {0, 0, 0, AP1}. Switch B finds that the
received configuration BPDU is superior to the
configuration BPDU of the local port {1, 0,1, BP1},
and updates the configuration BPDU of BP1.
zPort BP2 receives the configuration BPDU of
Switch C {2, 0, 2, CP2}. Switch B finds that the
configuration BPDU of the local port {1, 0, 1, BP2} is
superior to the received configuration BPDU, and
discards the received configuration BPDU.
BP1: {0, 0, 0,
AP1}
BP2: {1, 0, 1,
BP2}
Switch
B
zSwitch B compares the configuration BPDUs of all
its ports, and determines that the configuration
BPDU of BP1 is the optimum configuration BPDU.
Then, it uses BP1 as the root port, the configuration
BPDUs of which will not be changed.
zBased on the configuration BPDU of BP1 and the
path cost of the root port (5), Switch B calculates a
designated port configuration BPDU for BP2 {0, 5,
1, BP2}.
zSwitch B compares the computed configuration
BPDU {0, 5, 1, BP2} with the configuration BPDU of
BP2. If the computed BPDU is superior, BP2 will act
as the designated port, andthe configuration BPDU
on this port will be replaced with the computed
configuration BPDU, which will be sent out
periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated
port BP2:
{0, 5, 1, BP2}

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Device Comparison process BPDU of port
after
comparison
zPort CP1 receives the configuration BPDU of
Switch A {0, 0, 0, AP2}. Switch C finds that the
received configuration BPDU is superior to the
configuration BPDU of the local port {2, 0, 2, CP1},
and updates the configuration BPDU of CP1.
zPort CP2 receives the configuration BPDU of port
BP2 of Switch B {1, 0, 1, BP2} before the message
was updated. Switch C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP2}, and updates
the configuration BPDU of CP2.
CP1: {0, 0, 0,
AP2}
CP2: {1, 0, 1,
BP2}
By comparison:
zThe configuration BPDUs of CP1 is elected as the
optimum configuration BPDU, so CP1 is identified
as the root port, the configuration BPDUs of which
will not be changed.
zSwitch C compares the computed designated port
configuration BPDU {0, 10, 2, CP2} with the
configuration BPDU of CP2, and CP2 becomes the
designated port, and the configuration BPDU of this
port will be replaced with the computed
configuration BPDU.
Root port CP1:
{0, 0, 0, AP2}
Designated
port CP2:
{0, 10, 2, CP2}
zNext, port CP2 receives the updated configuration
BPDU of Switch B {0, 5, 1, BP2}. Because the
received configuration BPDU is superior to its old
one, Switch C launches a BPDU update process.
zAt the same time, port CP1 receives configuration
BPDUs periodically from Switch A. Switch C does
not launch an update process after comparison.
CP1: {0, 0, 0,
AP2}
CP2: {0, 5, 1,
BP2}
Switch
C
By comparison:
zBecause the root path cost of CP2 (9) (root path
cost of the BPDU (5) + path cost corresponding to
CP2 (4)) is smaller than the root path cost of CP1
(10) (root path cost of the BPDU (0) + path cost
corresponding to CP2 (10)), the BPDU of CP2 is
elected as the optimum BPDU, and CP2 is elected
as the root port, the messages of which will not be
changed.
zAfter comparison between the configuration BPDU
of CP1 and the computed designated port
configuration BPDU, port CP1 is blocked, with the
configuration BPDU of the port remaining
unchanged, and the port will not receive data from
Switch A until a spanning tree computing process is
triggered by a new condition, for example, the link
from Switch B to Switch C becomes down.
Blocked port
CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
After the comparison processes described in the table above, a spanning tree with
Switch A as the root bridge is stabilized, as shown in Figure 1-3.

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Switch A
优先级为0
Switch C
优先级为2
Switch B
优先级为1CP2
BP2
BP1
AP1
4
5
Switch B
with priority 1 CP2
BP2
BP1
AP1
4
5
Switch A
with priority 0
Switch C
with priority 2
Switch A
优先级为0
Switch C
优先级为2
Switch B
优先级为1CP2
BP2
BP1
AP1
4
5
Switch B
with priority 1 CP2
BP2
BP1
AP1
4
5
Switch A
with priority 0
Switch C
with priority 2
Figure 1-3 The final computed spanning tree
Note:
To facilitate description, the spanning tree computing process in this example is
simplified, while the actual process is more complicated.
2) The BPDU forwarding mechanism in STP
zUpon network initiation, every switch regards itself as the root bridge, generates
configuration BPDUs with itself as the root, and sends the configuration BPDUs at
a regular interval of hello time.
zIf it is the root port that received the configuration BPDU and the received
configuration BPDU is superior to the configuration BPDU of the port, the device
will increase message age carried in the configuration BPDU by a certain rule and
start a timer to time the configuration BPDU while it sends out this configuration
BPDU through the designated port.
zIf the configuration BPDU received on the designated port has a lower priority than
the configuration BPDU of the local port, the port will immediately sends out its
better configuration BPDU in response.
zIf a path becomes faulty, the root port on this path will no longer receive new
configuration BPDUs and the old configuration BPDUs will be discarded due to
timeout. In this case, the device will generate a configuration BPDU with itself as
the root and sends out the BPDU. This triggers a new spanning tree computing
process so that a new path is established to restore the network connectivity.
However, the newly computed configuration BPDU will not be propagated throughout
the network immediately, so the old root ports and designated ports that have not
detected the topology change continue forwarding data through the old path. If the new
root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur. For this reason, STP uses a state transition mechanism.

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Namely, a newly elected root port or designated port requires twice the forward delay
time before transitioning to the forwarding state, when the new configuration BPDU has
been propagated throughout the network.
1.1.2 Introduction to MSTP
I. Why MSTP
1) Disadvantages of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or
designated port must wait twice the forward delay time before transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port, which
directly connects to a user terminal rather than to another device or a shared LAN
segment.
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a
newly elected root port or designated port to enter the forwarding state much quicker
under certain conditions than in STP. As a result, it takes a shorter time for the network
to reach the final topology stability.
Note:
zIn RSTP, a newly elected root port can enter the forwarding state rapidly if this
condition is met: The old root port on the device has stopped forwarding data and
the upstream designated port has started forwarding data.
zIn RSTP, a newly elected designated port can enter the forwarding state rapidly if
this condition is met: The designated port is an edge port or a port connected with a
point-to-point link. If the designated port is an edge port, it can enter the forwarding
state directly; if the designated port is connected with a point-to-point link, it can
enter the forwarding state immediately after the device undergoes handshake with
the downstream device and gets a response.
Although RSTP support rapid network convergence, it has the same drawback as STP
does: All bridges within a LAN share the same spanning tree, so redundant links cannot
be blocked based on VLANs, and the packets of all VLANs are forwarded along the
same spanning tree.
2) Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and
RSTP. In addition to support for rapid network convergence, it also allows data flows of
different VLANs to be forwarded along their own paths, thus providing a better load
sharing mechanism for redundant links.
MSTP features the following:

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zMSTP supports mapping VLANs to MST instances by means of a
VLAN-to-instance mapping table.
zMSTP divides a switched network into multiple regions, each containing multiple
spanning trees that are independent of one another.
zMSTP prunes loop networks into a loop-free tree, thus avoiding proliferation and
endless recycling of packets in a loop network. In addition, it provides multiple
redundant paths for data forwarding, thus supporting load balancing of VLAN data
in the data forwarding process.
zMSTP is compatible with STP and RSTP.
II. Some concepts in MSTP
As shown in Figure 1-4, there are four multiple spanning tree (MST) regions, each
made up of four switches running MSTP. In light with the diagram, the following
paragraphs will present some concepts of MSTP.
C
A
B
D
BPDU BPDU
BPDU
Region A0
VLAN 1 mapped to instance 1
VLAN 2 mapped to instance 2
Other VLANs mapped CIST
CST
C
A
B
D
Region B0
VLAN1 mapped to instance 1
VLAN2 mapped to instance 2
Other VLANs mapped CIST
Region C0
VLAN 1mapped to instance 1
VLANs2 and 3mapped to instance 2
Other VLANs mapped CIST
Region D0
VLAN 1 mapped to instance 1,
B as regional root bridge
VLAN 2 mapped to instance 2,
C as regional root bridge
Other VLANs mapped CIST
Figure 1-4 Basic concepts in MSTP
1) MST region
An MST region is composed of multiple devices in a switched network and network
segments among them. These devices have the following characteristics:
zAll are MSTP-enabled,
zThey have the same region name,
zThey have the same VLAN-to-instance mapping configuration,
zThey have the same MSTP revision level configuration, and
zThey are physically linked with one another.
In area A0 in Figure 1-4, for example, all the device have the same MST region
configuration: the same region name, the same VLAN-to-instance mapping (VLAN1 is
mapped to MST instance 1, VLAN2 to MST instance 2, and the rest to the command

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and internal spanning tree (CIST). CISTrefers to MST instance 0), and the same MSTP
revision level (not shown in the figure).
Multiple MST regions can exist in a switched network. You can use an MSTPcommand
to group multiple devices to the same MST region.
2) VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes the
mapping relationships between VLANs and MST instances. In Figure 1-4, for example,
the VLAN-to-instance mapping table of region A0describes that the same region name,
the same VLAN-to-instance mapping (VLAN1 is mapped to MST instance 1, VLAN2 to
MST instance 2, and the rest to CIST.
3) IST
Internal spanning tree (IST) is a spanning tree that runs in an MSTP region, with the
instance number of 0. ISTs in all MST regions the common spanning tree (CST) jointly
constitute the common and internal spanning tree (CIST) of the entire network. An IST
is a section of the CIST in an MST region. In Figure 1-4, for example, the CIST has a
section is each MST region, and this section is the IST in each MST region.
4) CST
The CST is a single spanning tree that connects all MSTregions in a switched network.
If you regard each MST region as a “device”, the CST is a spanning tree computed by
these devices through MSTP. For example, the red lines in Figure 1-4 describe the
CST.
5) CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that
connects all devices in a switched network. In Figure 1-4, for example, the ISTs in all
MST regions plus the inter-region CST constitute the CIST of the entire network.
6) MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one
spanning tree being independent of another. Each spanning tree is referred to as a
multiple spanning tree instance (MSTI). In Figure 1-4, for example, multiple spanning
tree can exist in each MST region, each spanning tree corresponding to a VLAN. These
spanning trees are called MSTIs.
7) Regional root bridge
The root bridge of the IST or an MSTI within an MSTregion is the regional root bridge of
the MST or that MSTI. Based on the topology, different spanning trees in an MST region
may have different regional roots. For example, in region D0 in Figure 1-4, the regional
root of instance 1 is device B, while that of instance 2 is device C.
8) Common root bridge
The root bridge of the CIST is the common root bridge. In Figure 1-4, for example, the
common root bridge is a device in regionA0.

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9) Boundary port
Aboundary port is a port that connects an MST region to another MSTconfiguration, or
to a single spanning-tree region running STP, or to a single spanning-tree region
running RSTP.
During MSTP computing, a boundary port assumes the same role on the CIST and on
MST instances. Namely, if a boundary port is master port on the CIST, it is also the
master port on all MST instances within this region. In Figure 1-4, for example, if a
device in region A0 is interconnected with the first port of a device in region D0 and the
common root bridge of the entire switched network is located in regionA0, the first port
of that device in region D0 is the boundary port of region D0.
10) Roles of ports
In the MSTP computing process, port roles include designated port, root port, master
port, alternate port, backup port, and so on.
zRoot port: a port responsible for forwarding data to the root bridge.
zDesignated port: a port responsible for forwarding data to the downstream network
segment or device.
zMaster port: A port on the shortest path from the entire region to the common root
bridge, connect the MST region to the common root bridge.
zAlternate port: The standby port for a root port or master port. If a root port or
master port is blocked, the alternate port becomes the new root port or master
port.
zBackup port: If a loop occurs when two ports of the same device are
interconnected, the device will block either of the two ports, and the backup port is
that port to be blocked.
A port can assume different roles in different MST instances.
Figure 1-5 Port roles

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Figure 1-5 helps understand these concepts. Where,
zDevices A, B, C, and D constitute an MST region.
zPort 1 and port 2 of device A connect to the common root bridge.
zPort 5 and port 6 of device C form a loop.
zPort 3 and port 4 of device D connect downstream to other MST regions.
III. How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are
interconnected by a computed CST. Inside an MST region, multiple spanning trees are
generated through computing, each spanning tree called an MST instance. Among
these MST instances, instance 0 is the IST, while all the others are MSTIs. Similar to
RSTP, MSTP uses configuration BPDUs to compute spanning trees. The only
difference between the two protocols being in that what is carried in an MSTP BPDU is
the MSTP configuration on the device from which this BPDU is sent.
1) CIST computing
By comparison of “configuration BPDUs”, one device with the highest priority is elected
as the root bridge of the CIST. MSTP generates an ISTwithin each MSTregion through
computing, and, at the same time, MSTP regards each MST region as a single device
and generates a CST among these MST regions through computing. The CST and
ISTs constitute the CIST of the entire network.
2) MSTI computing
Within an MST region, MSTP generates different MSTIs for different VLANs based on
the VLAN-to-instance mappings.
MSTP performs a separate computing process, which is similar to spanning tree
computing in STP, for each spanning tree. For details, refer to “1.1.1 IV. How STP
works” in section 1.1.1.
In MSTP, a VLAN packet is forwarded along the following paths:
zWithin an MST region, the packet is forwarded along the corresponding MSTI.
zBetween two MST regions, the packet is forwarded along the CST.
IV. Implementation of MSTP on devices
MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be
recognized by devices running MSTP and used for spanning tree computing.
In addition to basic MSTP functions, many management-facilitating special functions
are provided, as follows:
zRoot bridge hold
zRoot bridge backup
zRoot guard
zBPDU guard
zLoop guard

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1.2 Configuration Task List
Before configuration, you need to know the position of each device in each MST
instance: root bridge or leave node. In each instance, one, and only one device acts as
the root bridge, while all others as leaf nodes.
Task Remarks
Configuring an MST Region Required
Specifying the Root Bridge or a Secondary Root
Bridge Optional
Configuring the Work Mode of MSTP Device Optional
Configuring the Priority of the Current Device Optional
Configuring the Maximum Hops of an MST
Region Optional
Configuring the Network Diameter of a Switched
Network Optional
Configuring Timers of MSTP Optional
Configuring the Timeout Factor Optional
Configuring the Maximum Transmission Rate of
Ports Optional
Configuring Ports as Edge Ports Optional
Configuring Whether Ports Connect to
Point-to-Point Links Optional
Configuring the MSTP Packet Format for Ports Optional
Configuring
the Root
Bridge
Enabling the MSTP Feature Required
Configuring an MST Region Required
Configuring the Work Mode of MSTP Optional
Configuring the Timeout Factor Optional
Configuring the Maximum Transmission Rate of
Ports Optional
Configuring Ports as Edge Ports Optional
Configuring Path Costs of Ports Optional
Configuring Port Priority Optional
Configuring Whether Ports Connect to
Point-to-Point Links Optional
Configuring the MSTP Packet Format for Ports Optional
Configuring
Leaf Nodes
Enabling the MSTP Feature Required
Performing mCheck Optional

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Task Remarks
Configuring Digest Snooping Optional
Configuring No Agreement Check Optional
Configuring Protection Functions Optional
Note:
If both GVRP and MSTP are enabled on a device at the same time, GVRP packets will
be forwarded along the CIST. Therefore, if both GVRP and MSTP are running on the
same device and you wish to advertise a certain VLAN within the network through
GVRP, make sure that this VLAN is mapped to the CIST (instance 0) when configuring
the VLAN-to-instance mapping table.
1.3 Configuring the Root Bridge
1.3.1 Configuring an MST Region
I. Configuration procedure
Follow these steps to configure an MST region:
To do... Use the command... Remarks
Enter system view system-view —
Enter MST region view stp region-configuration —
Configure the MST
region name region-name name
Required
The MST region name
is the MAC address by
default
instance instance-id vlan
vlan-list
Configure the
VLAN-to-instance
mapping table vlan-mapping modulo
modulo
Use either command
All VLANs in an MST
region are mapped to
MST instance 0 by
default
Configure the MSTP
revision level of the MST
region revision-level level Optional
0 by default
Activate MST region
configuration manually active
region-configuration Required
Display all the
configuration information
of the MST region
check
region-configuration Optional

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To do... Use the command... Remarks
Display the currently
effective MST region
configuration information
display stp
region-configuration
The display command
can be executed in any
view
Caution:
Two device belong to the same MST region only if they are configure to have the same
MST region name, the same VLAN-to-instance mapping entries in the MST region and
the same MST region revision level, and they are interconnected via a physical link.
Your configuration of MST region–related parameters, especially the VLAN-to-instance
mapping table, will cause MSTP to launch a new spanning tree computing process,
which may result in network topology instability. To reduce the possibility of topology
instability caused by configuration, MSTP will not immediately launch a new spanning
tree computing process when processing MST region–related configurations; instead,
such configurations will take effect only if you:
zactivate the MST region–related parameters suing the active
region-configuration command, or
zenable MSTP using the stp enable command.
II. Configuration example
# Configure the MST region name to be “info”, the MSTP revision level to be 1, and
VLAN 2 through VLAN 10 to be mapped to instance 1 and VLAN 20 through VLAN 30
to instance 2.
<Sysname> system-view
[Sysname] stp region-configuration
[Sysname-mst-region] region-name info
[Sysname-mst-region] instance 1 vlan 2 to 10
[Sysname-mst-region] instance 2 vlan 20 to 30
[Sysname-mst-region] revision-level 1
[Sysname-mst-region] active region-configuration
1.3.2 Specifying the Root Bridge or a Secondary Root Bridge
MSTP can determine the root bridge of a spanning tree through MSTP computing.
Alternatively, you can specify the current device as the root bridge using the commands
provided by the system.

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I. Specifying the current device as the root bridge of a specific spanning tree
Follow these steps to specify the current device as the root bridge of a specific
spanning tree:
To do... Use the command... Remarks
Enter system view system-view —
Specify the current
device as the root
bridge of a specific
spanning tree
stp [instance instance-id ] root
primary [ bridge-diameter
bridge-number ] [ hello-time
centi-seconds ]
Required
The device does not
function as the root
bridge by default
II. Specifying the current device as a secondary root bridge of a specific
spanning tree
Follow these steps to specify the current device as a secondary root bridge of a specific
spanning tree:
To do... Use the command... Remarks
Enter system view system-view —
Specify the current
device as a
secondary root
bridge of a specific
spanning tree
stp [instance instance-id ] root
secondary [ bridge-diameter
bridge-number ] [ hello-time
centi-seconds ]
Required
By default, a device
does not function as a
secondary root bridge
Note that:
zUpon specifying the current device as the root bridge or a secondary root bridge,
you cannot change the priority of the device.
zYou can configure the current device as the root bridge or a secondary root bridge
of an MST instance, which is specified by instance instance-id in the command. If
you set instance-id to 0, the current device will be the root bridge or a secondary
root bridge of the CIST.
zThe current device has independent roles in different instances. It can act as the
root bridge or a secondary root bridge of one instance while it can also act as the
root bridge or a secondary root bridge of another instance. However, the same
device cannot be the root bridge and a secondary root bridge in the same instance
at the same time.
zThere is one and only one root bridge in effect in a spanning tree instance. If two or
more devices have been designated to be root bridges of the same spanning tree
instance, MSTP will select the device with the lowest MAC address as the root
bridge.